Copper-nickel pipe is commonly used in ships because of its resistance to the corrosive effects of the salt air outside of the pipe and the corrosive effects of any salt water inside the pip. This copper-nickel pipe is generally thin walled and is subject to burn through if the welding temperature is too high. However, if the welding temperature is too low, the weld penetration and bonding will be inadequate, thereby necessitating that the weld be ground out and that portion of the joint be rewelded.
Tungsten inert gas (TIG) welding provides an arc which is concentrated on one spot. This allows the welding operator to cut the heat back and still maintain a good puddle and obtain good penetration and bonding. Metallic inert gas (MIG) welding provides an arc which is not as concentrated and therefore a high overall temperature is required in order to achieve a puddle which penetrates and bonds well to the pipe. If the welding power applied is too high, then the weld will penetrate through the joint. It is possible to cut the welding power down to the point where it is unlikely that the weld will penetrate through the pipe. However, in this case, the pipe will heat up very slowly, and the penetration and bonding may be inadequate for the first part of the weld. This requires grinding out the weld and rewelding the pipe.
Furthermore, if the welding operator should start off with a hotter weld, break the arc long enough to readjust the welding power, and then continue on the weld, the penetration on the first part of the restarted weld will generally be inadequate. This occurs because the heat conductivity of the pipe draws the heat away from the weld area very quickly. Therefore, when the welder continues the welding operation with the lower power, the degree of penetration and bonding will be inadequate and it will take a short amount of time before the pipe temperature heats up to the point where the degree of penetration and bonding is proper. Again, this causes a poor weld and that part of the weld must be ground out and that particular portion of the pipe rewelded. Because of the difficulties of MIG welding of copper-nickel pipe, TIG welding is preferred for this type of pipe, especially where out-of-position welding is being performed.
If the weld power could be controlled, so as to achieve adequate penetration at all times, while avoiding burning through of the pipe, MIG welding would be preferred over TIG welding because MIG welding operations proceed at five to six times the rate of TIG welding operations.
Therefore, there is a need for a MIG welder which provides a high starting power so as to achieve good penetration and bonding and then provides a lower, continuous welding power so as to prevent burning through the pipe.
Furthermore, there is a need for a MIG welder which monitors the temperature of the pipe and adjusts the welding power so as to achieve good penetration and bonding without burn-through.